The invention relates to a color display tube comprising, in a glass envelope with a substantially rectangular display window having a pattern of phosphors which can luminesce in different colors when struck by electron beams, a system of electron guns for generating a number of electron beams directed on the display window, and a substantially rectangular shadow mask comprising a mask sheet having an upright edge; and more particularly to such a CRT whose mask, within a blind edge, has a pattern of apertures, and which has weakened portions provided in the blind edge.
Such a color display tube is known from U.S. Pat. No. 3,809,945. This patent relates to a so-called delta tube in which the electron guns for the three electron beams are located at the corners of a triangle. Phosphor dots luminescing in the colors red, green and blue are provided on the display window in a pattern at the corners of triangles. A shadow msk is located inside the tube, a short distance from the display window. The mask is formed by a supporting frame and a mask sheet connected thereto by means of its upright edge. The mask sheet has, between the pattern aea and the upright edge, a blind edge which thus surrounds the pattern of circular apertures which associate each electron beam with luminescent phosphor dots of one color.
Such a mask sheet is manufactured from a flat steel plate in which the pattern of circular apertures is etched. The mask sheet is then deep drawn to its ultimate dish shape by means of a stretch forming process. In this process, the mask sheet is clamped along its circumference at the four rectangle sides and is stretched over a die. During and after stretching, the edge of the mask sheet is bent over. During stretch forming, the mask sheet material is stretched to beyond its elastic limit so that the mask sheet is permanently deformed.
Within the area of the pattern of apertures, the mask sheet is weaker than at the area of the blind edge. As a result of this sharp transition in tensile strength, the mask material, during the stretch forming process, is stretched excessively at the edge of the pattern of apertures, in particular, in the corners of the mask sheet. As a result, the apertures are deformed at the edge of the pattern. In the corners of the mask sheet, the material of the mask may even be stretched to such an extent that cracks occur in the mask material. In order to prevent this, weakened areas are provided around the pattern of apertures and form a transition between the blind edge having a comparatively large tensile strength and the pattern of apertures having a comparatively small tensile strength. The weakened portions are formed by a number of circular pits which form a continuation of the actual pattern of apertures. The width of the pattern of pits decreases from the corners towards the centers of the sides of the mask sheet.
In the usual present day color display tubes, the electron guns are no longer situated at the corners of a triangle but they are located in one plane (in-line). In these tubes, vertical phosphor lines capable of luminescing in the colors red, green and blue are provided alternately on the display screen. The shadow mask sheet has a pattern of rows of elongate apertures with small bridges between the apertures. Such a mask sheet has a much larger strength in the direction of the rows of apertures than in the direction at right angles to the rows of apertures. A stretch forming process as is used for mask sheets having circular apertures is consequently not suitable for mask sheets having rows of elongate apertures, because in such a process the bridges between the apertures would break.
A so-called bilateral or uniaxial drawing process is usually used for stretch forming mask sheets having rows of elongate apertures. Such a process is disclosed in German Patent Specification No. 2,628,894. During stretch forming the mask sheet, the two long rectangle sides are clamped between a drawing ring and a blank holder. These rectangle sides are perpendicular to the direction of the rows of apertures, that is to say perpendicular to the direction in which the mask sheet has its largest strength. On the short rectangle sides, the mask sheet is not clamped but there is some space between the blank holder and the drawing ring. During stretch forming over a die, the mask material is stretched in the direction of its largest strength. In the direction of the smaller strength the mask material is stretched to a much smaller extent, due to the permitted slip of the short rectangle sides between the drawing ring and the die.
As a result of this bilateral stretch forming process, however, undulations or wrinkles occur in the corners of the blind edge of the mask sheet which influence the pattern of apertures. This formation of wrinkles occurs because, in the bilateral stretch forming process, as contrasted with the unilateral stretch forming process, no excessive tensile loads but an excessive buckle load occurs in the corners of the mask sheet. This buckle load is such that wrinkles are formed which extend parallel to the diagonal in the each corner of the mask sheet. These undulations influence the pattern of the rows of elongate apertures. The apertures situated on these undulations thus have a different distance to the phosphor lines on the display screen and have a different effective area with respect to the electron beams. These differences cause a reduced landing tolerance and color defects.